Comparison of the ligating properties of disulphides and thioethers: dimethyl disulphide, dimethyl sulphide, and related ligands

Abstract

The stability constants of 1 : 1 complexes between dimethyl disulphide (dmds) or dimethyl thioether (dms) and Ca²⁺, Zn²⁺, Cd²⁺, Pb²⁺, or Ag⁺(K_ML for Mⁿ⁺+ L ⇌[ML]ⁿ⁺) have been determined in aqueous solution by ¹H n.m.r. shift measurements. The results [e.g. log K_Ca(dmds)ca.–1.4, log K_Cd(dmds)ca.–1.4, log K_Ag(dmds)= 2.01 ± 0.09; log K_Ca(dms)ca.–1.6, log K_Cd(dms)=–0.3 ± 0.2, and log K_Ag(dms)= 3.7 ± 0.3] show that the complexes with soft metal ions are stronger than those with borderline or hard metal ions. It is also evident that the ligating properties of the thioether moiety are somewhat more pronounced towards borderline and soft metal ions than those of the disulphide group. Spectrophotometrically determined stability constants (in 50% aqueous ethanol) for the complexes between Mn²⁺, Cu²⁺, or one of the above metal ions and dmds, diethyl sulphide, or tetrahydrothiophen accord with this. In addition, for several complexes of tetrahydrothiophen-2-carboxylate (thtc^–) and 1,2-dithiolan-3-carboxylate [= tetranorlipoate (tnl^–)], the dimensionless constants for the intramolecular equilibrium between the chelated isomer (which is bonded to the metal by the sulphur atom and the carboxylate group) and the simple carboxylate-co-ordinated isomer have been calculated, together with the percentages of the chelated isomer {e.g. 93 ± 1 for [Cu(thtc)]⁺ and 41 ± 7 for [Cu(tnl)]⁺; 55 ± 5 for [Cd(thtc)]⁺ and ⩽20% for [Cd(tnl)]⁺; ⩽20% for both [Mn(thtc)]⁺ and [Mn(tnl)]⁺}. Possible biological implications of such weak interactions and the resulting intramolecular equilibria are briefly discussed.